It is very difficult to accurately estimate wind speeds in terms of kilometres per hour. There are often claims of very strong winds, e.g. 80km/h. Without a lot of experience these estimates will be highly unreliable.
There is however a way by which you can visually determine the wind speed and accurately communicate it. It’s called the Beaufort scale. The Beaufort scale dates back to 1805 and was created by Francis Beaufort of the Royal Navy. It uses 13 numbered divisions to describe the wind speed by means of its effects.
To describe the wind speed, instead of trying to guess the speed in kilometres per hour, just estimate the Beaufort Scale number from the effects you’re observing. It may take a little while to learn the scale, but once you do it’ll be very useful for visual observations and will make your reports more accurate.
Here is a table of the Beaufort Scale. For simplicity I’ve only included land conditions. (Source: Wikipedia License: CC-ShareALike)
|Beaufort Number||Description||Wind Speed||Land Conditions|
|0||Calm||Under 1 km/h||Calm. Smoke rises vertically|
|1||Light Air||1.1 to 5.5 km/h||Smoke drift indicates wind direction. Leaves and wind vanes are stationary.|
|2||Light breeze||5.6 to 11 km/h||Wind felt on exposed skin. Leaves rustle. Wind vanes begin to move.|
|3||Gentle breeze||12 to 19 km/h||Leaves and small twigs constantly moving, light flags extended.|
|4||Moderate breeze||20 to 28 km/h||Dust and loose paper raised. Small branches begin to move.|
|5||Fresh breeze||29 to 38 km/h||Branches of moderate size move. Small trees begin to sway.|
|6||Strong breeze||39 to 49 km/h||Large branches in motion. Whistling sound heard in overhead wires. Umbrella use becomes difficult. Empty plastic bin tip over.|
|50 to 61 km/h||Whole trees in motion. Effort needed to walk against the wind.|
|62 to 74 km/h||Some twigs broken from trees. Cars veer on road. Progress on foot is severely impeded.|
|9||Strong gale||75 to 88 km/h||Some branches break off trees and some small trees blow over. Construction/temporary signs and barricades are blown over.|
|89 to 102 km/h||Trees are broken off or uprooted, structural damage likely.|
|11||Violent storm||103 to 117 km/h||Widespread vegetation and structural damage likely.|
|12||Hurricane Force||Over 118 km/h||Severe widespread damage to vegetation and structures. Debris and unsecured objects are hurled about.|